Paving machine control system



April 17, 1962 Filed Feb. 27. 1958 R. P. SHEA PAVING MACHINE CONTROL SYSTEM 2 Sheets-Sheet 1 .BEEF-'ORD l? SHEA.

April 17, 1952 R. P. SHEA PAVING MACHINE CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Feb. 27, 1958 Heeze/QD ,R SHEA) INVENTOR.

nitcd States Pate? 3,029,716 PAVING MACHINE CGNTROL SYSTEM Reeford P. Shea, Riverside, Calif., assigner to Preco Incorporated, Los Angeles, Calif., a corporation of California Filed Feb. 27, 1958, Ser. No. 717,874 7 Claims. (Cl. 94-46) This invention has to do generally with paving machines for depositing a mat of paving material on a roadway or the like. Such machines ordinarily include manual adjustments at each side of the machine for varying the level of the mat surface that is produced.

The invention relates more particularly to control means for such paving machines for facilitating control of the lateral or transverse slope of the resulting mat surface. The invention may provide to the operator an indication to aid him in manual control, or may provide means for automatically controlling the lateral slope of the mat surface. Such fully automatic control has the great advantage of freeing the operator of the machine to give his full attention to manual control of other aspects of the operation such as the general thickness of the mat.

The invention relates more particularly to paving machines which deposit the paving material adjacent the leading edge of a horizontal screed which extends transversely of the machine and is supported by the freshly laid mat. The level of the mat surface produced by such machines is typically controlled by variation of the inclination of the screed in the direction of travel. Manual adjustment means for varying the screed inclination are ordinarily provided at both sides of the machine, that is, at both ends of the screed. The screed is torsionally exible about its longitudinal axis sufficiently to permit different settings at the two ends of the screed. In the normal operation of such machines, the operator varies those adjustments simultaneously in the same direction to control the general thickness of the mat, and varies them differentially to control the lateral slope of v the mat surface.

accordance with a control signal that is related in a particular manner to the existing condition of adjustment of the screed. That relation is such that the machine not only detects and corrects errors of transverse slope of the mat currently being laid, but detects and corrects conditions of screed adjustment that would, if not corrected, cause errors of transverse slope to develop as the machine progresses along the roadway. By eliminating such errors of screed adjustment before the resulting slope errors actually appear in the pavement being produced, the control system of the invention produces a remarkably accurate and uniform surface.

Those and other objects of the invention are attained by sensing mechanism which is jointly responsive to the transverse slope of the mat surface currently Abeing laid down and to a condition of the machine which determines the rate of change of that transverse slope as the machine progresses. That sensing mechanism develops a control signal which represents a predetermined combination of the current mat slope and the rate of change of that slope. Such a signal provides, in effect, an indication of the transverse slope of the mat surface that will `be laid at some position in advance of the existing mat. By deriving a control signal which thus represents the projected transverse slope of the mat, rather than merely the present value of the transverse slope of the mat, the invention provides a type of control that tends to anticipate errors lice of screed adjustment before the resulting slope errors actually appear in the mat surface. When such tendency to produce an error in slope is detected by the sensing mechanism, the machine adjustment is immediately corrected, and the slope error is thereby prevented from actually occurring. A further advantage of the described anticipatory action is that any correction that may be required is made smoothly and gradually, so that no sharp changes in mat level are produced.

The described type of control signal, representing the projected mat slope, can be produced, in accordance with the present invention, by mechanism that is responsive jointly to the inclination of the length of the screed with respect to gravity, and to the difference in angle of attack at the two ends of the screed. The inclination of the length of the screed provides a measure of the transverse slope of the currently laid mat. And the differential angle of attack at two dongitudinally spaced points of the screed provides a measure of the rate at which the mat slope changes as the machine progresses. If the angle of attack at the two ends of the screed, for example, is not the same, that is, if the screed is twisted about its longitudinal axis, the actual mat slope typical-1y changes as the machine progresses, so that the projected mat slope differs from the actual or current mat slope.

A convenient and eiiiective mechanism for producing the described type of control signal in accordance with the invention effectively projects the screed surface forwardly at two points adjacent the respective ends of the screed, and compares the "levels of corresponding points of the two projections at a selected distance from the edge of the screed. That comparison may be utilized directly to produce the control signal. The screed adjustment may then be driven by suitable servo mechanism under control of the described signal.

The invention further provides means for comparing the projected mat slope with theparticular value of slope ang-le that is desired, and developing a nal control signal that represents directly the departure of the projected'mat slope from the desired value. ln preferred 4form of the invention, the desired value of transverse slope is supplied to the machine by means of an electrical signal, which may be adjusted, for example manually, during operation of'the machine. By varying that adjustment gradually as the machine progresses, the operator may therefore produce a corresponding gradual variation of transverse slope, as is typically required, for example, in paving the approaches to a super-elevated curve of highway.

In accordance with a `further aspect of the invention, a control signal of the type described may be displayed by any convenient type of indicating means, and used by the machine operator as a guide for manual adjustment of the screed.

A full understanding ofthe invention and of its further objects and advantages will be had from the yfollowing description of an illustrative manner in which it may be carried out. The particulars of that description, and of the accompanying drawings which form a part of it, are intended only as illustra-tion of the invention and not as a limitation upon its scope, which is defined in the appended claims.

ln the drawings:

FIG. l is a side elevation of a typical paving machine incorporating an illustrative embodiment of the inven- 5 tion;

FIG. 6 is a detail corresponding to a portion of FIG. l at enlargedl scale and partially broken away; and

FIG. 7 is a fragmentary rear elevation at enlarged scale and partially broken away.

In the present illustrative embodiment the invention is utilized for controlling a typical paving finishing machine of `known type. In that machine, `as illustratively and somewhat schematically represented, the main body 20 of lthe machine is supported on *two endless tractor treads 2-8 on a roadway base 29. T'hose treads are driven by an engine 30 via suitable reduction gear transmission and clutch mechanism, not specifically shown, under manual control of an operator -in seat 32.

At the forward end of machine 20 is a transverse roller 34 adapted to push ahead of the machine a truck loaded with paving material, indicated at 33. A receiving hopper 26 is provided in the forward portion of the machine in position to receive paving material 27 from the truck and to supply it to suitable conveying and distributing mechanism within the body of the machine. That mechanism is indicated generally by the numeral 24, and includes the power-driven transverse distributing screws 25. Those screws spread a loose pile 35 of paving material across roadway base 29, just to the rear of treads 28 and typically extending transversely the entire width of the machine.

Paving material is spread and tamped to an accurately controllable level to lform a compacted mat 36. That is accomplished by cooperative `action of the vertically reciprocating ramping bars l42 and the screed 44. The latter is a lgenerally horizontal plate extending transversely Y immediately behind the tamping bars. Each stroke of tamping bars 42 strikes off a limited amount of paving material and compacts it to uniform density directly adjacent the leading edge 45 of the screed. As the machine moves forward, screed 44 is drawn over the freshly formed mat, further smoothing and levelling its upper surface.

The term paving material is employed in the present specification and claims to refer to any material that can be handled in the described manner. Such material may comprise broken rock or gravel, for example, or bituminous mixes adapted `for producing the first course or the finishing course of a pavement. 'Ihe base on which the lmachine operates may comprise an actual roadway, or a surface of any other *type on which a course of material is to be laid. The termroadway will be employed ordin'arily in the generic sense of including any surface on which material is to be spread.

An 'important -feature of the type of machine with which the present invention is primarily concerned is the manner in which the thickness of the mat, or, more precisely, the level of its top surface is determined. The entire 'screed assembly, designated generally by numeral 40 and 'comprising the screed itself, tamping bars 42 and the eccentric mechanism for reciprocating the latter, is supporteddirectly on the top surface of the freshly laid mat.' 'The screed assembly is coupled to the main body 20 of the machine by coupling means which permit it to vary in vertical level and thus to tioat on theiinished mat while following at a fixed distance behind they machine tractor. Tamping bars 42 are driven `from engine 30 by transmission means indicated at 43 adapted to aclconlrnodate vertical movement of the screed assembly. The mat is produced by depositing and compacting the paving material immediately in front of the leading edge 45 'of the screed 44, and the freshly laid mat is further compacted and smoothed as the screed moves over it. The level of each new portion of the mat surface may 'be considered to be determined jointly by the level of the 4ina-t portion just previously laid, upon which the main body of the screed is supported, and upon the inclination angle of the screed surface in the direction of travel. The latter angle, indicated at 48 in FIG. l, will be referred to as the angle of attack of the screed, by analogy with the `angle of attack at which the airfoil of an aircraft engages the (supporting air. Variation of the angle of attack of the screed does n ot produce an abrupt change in the level of the mat surface, but causes a gradual transition from one level to another. For Iany particular conditions of operation, including a Yparticular paving material, there is typically an equilibrium value of the angle of lattack for which the mat surface extends horizontally in the direction of travel. That value of the angle of attack Will be referred to for convenience as the criticalk value.

It is usual in paving machines of the present type to make the -screed torslonally flexible about its longitudinal axis, and to provide independently operable means for adjusting the angle of attack of its respective end portions. The mat surface level is then separately adjustable along the two side edges `of the mat. vManipulation of those two adjustments in suitable mutual relationship permits the operator to control the transverse slope of the resulting mat. For example, by increasing the vangle of attack at vboth ends of the screed, the mat thickness may be caused to increase substantially uniformly across the mat; and by increasing the angle of attack at one end of the screed and decreasing -it at the other end the transverse slope of the surface can be varied without substantially affecting the average thickness of the mat.

The coupling means between the tractor and the screed assembly is ordinarily of a type which produces a ce1tain degree 'of regulation of the angle of attack of the screed. ln the present illustrative machine, that coupling means is of a Well known type which comprises two rearwardly extending `draft arms Si) and 51 which 'are independently pivotally mounted on opposite sides of tractor 2G. With such coupling, vertical movements of the tractor itself, whether in response to irregular or intended changes in level of the roadway, cause Vcorresponding but relatively small changes in the angle of attack on the screed. Mechanism -is also provided for manually mtering the angle `of attack at each end of the screed., That is done on some paving machine by pivoting the forward ends of draft arms Sti and 51 at points which are vertically adjustable with respect to fthe tractor body. In the present illustrative type ofmachine the draft arms are pivoted on a fixed trunnion axis 52,` and adjustment of the angle of attack is accomplished by structure which permits" adjustable variation ofthe angle `between the screed surface and the length of each arm. The screed side frames 46,k to which the screed ends are iixedly connected, are typically mounted at a fixed average distance below the respective draft -arrn's by vertical supports, indicated -at 54; and are rotatably adjustable with respect to supports 54 about a transverse axis 55, as by the generally vertically screws 56 and 57. rl`he lower end of each screw is rotatably mounted on a bracket member 58 whic his universally related to the screed side frame rearward of axis 55; and the upper part of the screw is threaded in a bracket member 59 which is universally related to the end of the draft arm. Screw rotation thus varies the angle between the length of the draft arm and the plane of the screed surface adjacent its end.

With the described type of coupling between screed assembly and machine proper, the angle of attack of each end of screed 44 is determined substantially independently by two primary factors: the inclination of the associated draft arm, and the condition of adjustment of screw 56 or 57. For a given screw adjustment and type of pavmg material, and with the tractor moving over a flat horizontal roadway, there is a definite equilibrium mat thickness at which the angle of attack of the screed has the above defined critical value. lf the mat initially has a different thickness at one or both ends, the surface level of the lmat laid down tends to increase or decrease progressively, causing one or both of the draft arms to swing about trunnion ,axis 52, until the angle .of attack at both ends of the screed reaches the critical value at which the machine produces a horizontal rnat surface. If the settings of screws 56 and 57 are different, the resulting equilibrium mat level is correspondingly different at the two ends of the screed, that is, at the two transverse edges of the mat, so that the mat surface has a'transverse slope. That equilibrium condition typically'continues until disturbed either by variation in level of the roadway base, which causes one or both trunnions to rise or fall, or by a change in adjustmentof one or both of the control screws 56 and 57. Either type of change shifts the angle of attack away from its critical or equilibrium value at one or both ends of the screed; and the mat level again changes progressively until equilibrium conditions are reestablished.

For example, if one track of machine 2i) encounters a susbtantial depression, the trunnion on that side is lowered, reducing the angle of attack of that end of the screed. If no compensating adjustment is made, the mat level at that side gradually decreases toward a level at which the angle of attack again equals the critical value. By suitable adjustment of the appropriate control screw, the angle of attack can, at least in theory, be maintained continuously at the critical value, regardless of vertical movement of the trunnions. It is thus possible to produce a mat surface that does not reflect irregularities in the roadway base. In performing such adjustment, it is necessary to distinguish between temporary deviations from level of the road base and intentional variations.

Such adjustment is rendered diflicult by the same characteristic of the machine that tends inherently to smooth out errors in the roadbed. By the time an error in mat level has become noticeable, the error in slope of the mat surface in the direction of travel is typically so great that it can only be corrected over an appreciable distance;

. and throughout that distance the error in level continues to increase. It is therefore essential, so far as possible, to detect and correct errors in angle of attack of the screed even before they produce a measurable error in level of the mat surface.

The present invention provides structure whicheffectively projects or produces the screed surface parallel to itself forward of the leading edge of the screed for a definite predetermined distance at each end of the screed, and provides a comparison of the level of the two projections. As illustrati-vely shown in FIGS. l to 4, the projection mechanism for accomplishing that purpose comprises the arms 7! and 71, which are fixedly mounted directly on the respective screed side frames 46, and which project forwardly for a predetermined distance typically equal to several feet. The arms 7S, the screed side frames 46 and the ends of the screed itself are thus'rotatable as a unit about pivots 55 inV response to movement of screws 56 and 57. Arms 70 are preferably guided by guide bars 72 and 73 which are attached in spaced relation to draft arms 50 and 5l, respectively, providing vertical guide channels in which the forward portions of the arms 70 and 71 may swing freely about screed axis 55 in response to adjustment of screws 56 and 57.

The geometrical forward projections of the screed surface adjacent its ends are indicated schematically by the line 76 in FIGS. l and 6, which is a linear continuation of the screed surface beyond its leading edge 45. The forward portions of arms 70 and 7l are provided with bracket formations 74 and 75, respectively. Those formations lie' in effect directly above substantially fixed points 80 of geometrical projections 76, at vertical distances indicated by the dashed line 78. Those vertical distances 78 are substantially constant and preferably equal. Screed rotation causes slight variations in the vertical distances '78 and also in the distances of points Si) from the screed. However, such variations are of second order compared to the distances themselves, and may be neglected in practice. In full effect, the two bracket formations 74 and 75 are movablejvertically in direct correspondence with the the slope error.

vertical movement of respective definite points 8i) of the actual screed projections. Hence the desired signal can be obtained by comparing the levels of formations 74 and 75. For the present purpose it is not necessary that those formations lie directly in the geometrical projections 76 of the screed surface, which would be inconvenient under some conditions of operation;

In the present embodiment, the relative level of the two formations 74 and '75 is sensed by structure which comprises the gantry beam 84 and means responsive to the angular relation of that beam to the direction of gravity. Gantry beam 84 extends transversely across the main body of machine 2t), typically forward of engine Si?, and has its respective ends mounted on the formations 74 and 75, as by means of the pivot structures 82 and S3 which permit limited rotary movement about respective transverse axes indicated at 85 and 85a. Gantry beam 84 is maintained in a substantially vertical plane, as by the generally horizontal link 86, the forward end of which is pivoted at S7 to an intermediate point of the beam, and the rearward end of which is pivotally mounted at 38 on a bracket 89 fixedly mounted on machine 20. The described pivotal connections at 82, S3, 87 and SS are so constructed yas to permit the very small amount of univers'al rotational freedom required to accommodate the maximum anticipated differential movement of the two ends of the screed.

Sensing means responsive to inclination of gantry beam $4 typically comprise a pendulum 90 which may be mounted directly on the beam, as in a pendulum case indicated at 92. The axis 94 of pendulum 90 is substantially parallel to the direction of travel of the machine. Means of any suitable type are preferably provided for adjusting the position of pendulum case 92 about axis 94, as by the threaded mounting studs and adjustable nuts indicated at A visual signal may be derived Afrom the relative rotational position of case 92 and pendulum 90 by any convenient mechanism, indicated illustratively as the pointer and scale 91 y(FIG. 2). As an alternative or a `supplement to such visual indication, an electrical signal is typically developed by a suitable type of transducer, shown typically as a potentiometer 95 (FIG. 3). An arcuate potentiometer winding 96 is mounted in xed relation to the case, and a potentiometer brush 97 is carried by pendulum 90 and Sweeps over the winding in respouse to pendulum movement. Electrical connection to brush 97 may be provided by a flexible lead, not explicitly shown. That lead, like the leads to winding 96, passes in insulated relation through the wall of case 92. Suitable damping means are preferably provided for the pendulum movement, such as damping fluid filling the entire interior of case 92.

The electrical signal derived from transducer 95 is typically compared electrically with a suitable signal of similar type representing the desired transverse slope of the mat surface, to derive a difference signal representing That difference signal may then be displayed visually, or may be utilized in any suitable type of servo mechanism to control power means for driving the angle of attack adjustment at one end of the screed in the appropriate direction to reduce the error signal. Such power means typically comprises an electric motor, indicated schematically at 120, which is coupled by suitable speed reduction mechanism to one of the adjusting mechanisms, whatever their nature; those mechanisms being represented in the present embodiment by the screws 56 and 57. A motor may, for example, be provided for each screed adjusting screw, with switching mechanism for manually selecting the desired screw for automatic drive during each paving run.

An illustrative servo system is indicated schematically in FIG. 4. In that system, the output signal from potentiometer 95 is compared with the output signal from a control potentiometer 105, which may be adjusted manually by the handle 104 to correspond to the` desired valuel of the transverse mat slope. -Potentiometer 105 is preferably mounted in some conveniently accessible position on machine 20, as in a cabinet 103 which is carried by the screed assembly (FIG. '1). Control handle 104 may be provided with a scale calibrated directly in terms of mat slope to facilitate adjustment of potentiometer 105. A source of electrical power is indicated as the battery 1%, connected in series with a main control switch 102. Winding 96 of pendulum potentiometer 95 isconnected in parallel with the winding 106 of control potentiometer 195 across battery 100 to form a bridge circuit. Small adjustable resistances 1108 and 1&9 are preferably provided as indicated to facilitate balancing the bridge and adjusting its operation. The voltage signal tapped from potentiometer 95 by brush 97 and the voltage signal tapped from potentiometer 105 by the brush 107 are supplied via the respective lines 110 and 111 as input to the servo amplifier 115. Amplifier 115 is supplied with electric power as via the lines 112 from battery 106, and produces an output signal on lines 116. That output signal may be of any suitable type to drive the reversible motor 120. Servo amplifier 115, which may be of known type, responds to zero voltage difference between input lines 110 and 111 by supplying zero signal to motor 126; and responds to voltage differences between the input lines by supplying motor 120 with signals which drive it in one direction for one polarity of input signal and in the other direction for the opposite polarity of input signal. The wiring and mechanical connections are so arranged that the direction of motor drive tends in each instance to change the inclination of gantry beam S4 in the direction which reduces the amplitude of the input signal.

Amplifier 115 is preferably provided with suitable out- 'i put connections to control a meter 125, the reading of which indicates the size and polarity of the input error Signal on lines 110 and 111. Meter 125 may be mounted on cabinet 163. It provides visual indication of the condition of adjustment of the screed, which may be used `as a guide for manual adjustment when servo motor 120 is omitted or is not used. Meter 125 is also useful in aligning and checking the apparatus.

The most favorable projection distance from the screed` forward to the points 80` at which the described comparison of level is made is usually of the same order as the distance from the screed to trunnion axis 52. Mechanism of any suitable type is preferably provided for permitting convenient adjustment of the projection distance. In the present illustrative embodiment, a plurality of pivot mounting formations are provided at longitudinally spaced points of arms 70 and 71, as indicated at 74, 74a and 74h in IFIG. 1. The desired effective length of arm 70 may then be obtained by pivoting gantry beam 84 at the appropriate formation at each of its ends. The effective length of support link 86 is adjusted correspondingly, as by shifting the pivot 88 among the plurality of pivot fittings provided in the link, as at 88a and 8gb.

The arms'70 and 71 are illustrative of structure of any suitable-type mounted on the respective sides of the machine, and coupled to the screed or its-mounting and control mechanism in yany suitable manner which causes a formation, such a's the bracket formations 74 and 75, to move vertically in direct correspondence with the vertical movement of selected points, such as points 80 and 81 of the forward projections of the respective end portions of the screed surface. For example, arms may be pi-votally mounted on the respective draftarms 50 and S1 and caused to swing about their pivots in accordance with the rotary movement of the Vrespective' screed end portions vabout-pivot axis 55. An Yillustrative arrangement of that type is shown schematically in FIG. 5.

Right arm 70a is there shown pivoted on draft arm '50 at 124. The main portion of the arm extends forward from pivot 124 to formation 74e at the end of the arm; and a short actuating arm 70bl extends rearwardly from pivot '124. -Arm 70b is linked to Athe `screed `frame at the right end of the screed, as by a generally `vertical link 125, which is pivotally related to actuating arm 70b at @126 and to the screed :frame near its rearward edge at V127. 1f, for example, the horizontal distance between pivots 124 and 126 is the same as the distance between pivot 127 and' main screed pivot 55, then the angular rotation of arm 76a about its mounting pivot 12.4 is substantially the same as that of the end portion of the screed frame about its pivot 55. -With that arrangement, the movement of formation 74e at the forward end kof arm 7dr: will be seen to be essentially the same .as that of formation 74 in the previously described embodiment. Sensing means may be provided responsive to the relative level of formation 74e and of the corresponding formation on the other side of the machine. Such sensing means may, for example, comprise a gantry beam and pendulum mechanism such as that already described. Such sensing means will be capable of developing a control signal of the type already described.

I claim:

1. In combination with apaving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portion mounted on the respective draft screed, Vand means controlled jointly by the-settable -means and the sensing means and producing a signal thatjrepresents the difference between said vdesired inclination of the mat surface and said inclination of the screed proj ection, and power means for driving one of said adjusting means in response to said signal to maintain the desired transverse slope of the mat surface.

2. In combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally exible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means vfor depositing paving material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the l, deposited material; a structure at each end of the screed,

means defining a reference point on each structure forwardly of the screed, means coupling said structures to the screed whereby said reference points move vertically in accordance with the respective points -of the forward projection of thek screed that are substantially equally spaced forward of the screed and are transversely spaced from each other, .and sensing means .interconnecting said reference points vand producing a signal that represents the difference in elevation of said reference points for facilitating manual operation of said adjusting means to maintain the desired transverse slope of the mat surface.

3. In combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot Vaxes and extending rearwardly therefrom, anelongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed to .form amat on which the .screed rides, and-adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; a structure at each end of the screed, means defining a reference point on each structure forwardly of the screed, means coupling said struca desired transverse slope of the mat surface.

4. In combination with a paving machine that is movable forwardly over the ground and that comprises two Vdraft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rear- Wardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on ,whichthe screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited materal; forwardly extending arms pivotally mounted on the respective dra-ft arms, means on the arms dening respective reference points at substantially equal distances forward of the arm pivots, means coupling the arms and the screed for driving rotation of the arms in accordance with the respective screed end por-tions, and sensing means interconnecting said reference points and producing a signal that represents the difference in elevation of said reference points for facilitating manual operation of said adjusting means to maintain the desired transverse slope of the mat sur- 5. The combination defined in claim 4, wherein said sensing means comprises a substantially rigid elongated member extending transversely of the machine with its end portions supported at ythe respective reference points, and means responsive to the longitudinal inclination of said member. Y

6. in combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides ofthe machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed -to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the respective screed end portions engage the deposited material; forwardly extending arms pivotally mounted on the respective draft arms, means on the arms delining respective reference points at substantially equal distances forward of the arm pivots, means coupling the arms and the screed for driving rotation of the arms in accordance with the respective screed end portions, and power means responsive to differences in elevation of the two reference points to automatically control one of said adjusting means to maintain a desired transverse slope of the mat surface.

7. in combination with a paving machine that is movable forwardly over the ground and that comprises two draft arms pivotally mounted at opposite sides of the machine on transverse pivot axes and extending rearwardly therefrom, an elongated, torsionally flexible screed having its end portions mounted on the respective draft arms adjacent their rearward ends, means for depositing paving material adjacent the forward edge of the screed to form a mat on which the screed rides, and adjusting means operable to vary independently the angles of attack at which the lrespective screed end portions engage the deposited material; means adjustably settable to represent the desired transverse inclination of the mat surface, setting means coupled to the screed and responsive to the difference in television of the two ends of the screed and also to the difference between -the angles of attack at the t-Wo ends of the screed, means controlled jointly by the settable means and the sensing means and producing a signal that represents the difference between the desired inclination of the mat surface and the said response of the sensing means, and power means for driving one of said adjusting means in response to said signal to maintain the desired transverse slope of the mat surface.

References Qited in the tile of this patent UNITED STATES PATENTS 1,936,518 -McColm Nov. 21, 1933 2,138,828 Barber Dec. 6, 1938 2,292,241 Reeves Aug. 4, 1942 2,295,519 Millikin et al. Sept. 8, 1942 2,491,275 Millikin Dec. 13, 1949 2,591,502 Bohannan et al. Apr. l, 1952 2,796,685 Besinger June 25, 1957 2,846,635 Shea Aug. 5, 1958 2,864,452 Guntert et al Dec. 16, 1958 UNITED STATES PA CERTIFICATE OF TENT OFFICE CORRECTION Apr-11 17, 1962 Patent No.

Reeford P. Shea It is hereby certified that error appears in the above numbered patent requiring correction and that the sa corrected below.

id Letters Patent should read as In the grant,

line l, for' 'f N Reeford P Reeford F. Shea" read Shea column 8, line 26, for "portion" read portions line 31, for "creed" read screed column lO, line 27, for "setting" read sensing line 28, for "television" read elevation (SEAL) Attest:

ERNEST w. swIDEE DAVID L- LADD Attesting Officer Commissioner of Patents 

